1. Field of the Invention
The invention is directed to an optical system for a fundus camera used for the observation and imaging of the ocular fundus. When the ocular fundus is imaged with a camera of this kind, there are generally reflections at the cornea and at surfaces of the imaging optical system which have a negative effect on the quality of the images and the evaluation thereof.
2. Description of Related Art
A fundus camera basically comprises a multi-stage optical system in which an opthalmoscope lens generates an intermediate image that is imaged by a tracking system or main objective onto a film, onto a CCD matrix of a CCD camera, or also in an intermediate image for visual observation with an ocular. In this system, the opthalmoscope lens is also a component part of the elements of the illumination system.
Reflections at the cornea and at the surfaces of the opthalmoscope lens pose a special problem in the observation and recording of the fundus because the light which is reflected by the retina and which carries the information of actual interest has a substantially lower intensity than the light which is reflected before entering the eye. Interfering cornea reflections are usually prevented by partitioning the pupil of the eye. For this purpose, the opthalmoscope lens images an illumination ring in the eye pupil. The illumination beams reflected at the cornea miss the aperture of the observation system. Only the area inside the illumination ring is used for observation.
DE-OS 35 19 442 describes an optical system in which light components which could enter the observation aperture through reflection at the opthalmoscope lens or at the cornea are blocked out by means of “black point plates” which are arranged at a suitable location in the beam path and which are coated in a defined manner by light-absorbing layers. This type of reflection suppression has come to be known as “anti-reflection point.” A disadvantage of this concept is the proximity of the anti-reflection point to the field diaphragm. The absorption of the individual light components can become visible as irregular illumination of the fundus. Ring-shaped shadows occur which impair the image impression and, therefore, impede evaluation by the doctor.
DE 103 16 416 describes an optical system in which a multilens objective is provided in place of the opthalmoscope lens. The lenses of the multilens objective are tilted relative to one another in such a way that direct reflections at the interfaces do not enter the aperture of the observation system. Tilting in the x-direction and y-direction is provided for each pair of lenses. This optical system requires a substantial outlay for mechanical mounts for the individual lenses of the objective. Further, transverse chromatic aberrations and longitudinal chromatic aberrations are produced and must be compensated at great expense in the downstream optics system, both in the observation portion of the beam path and in the illumination portion of the beam path. In applications with very small beam diameters, e.g., in laser applications, the large quantity of optical interfaces and the long glass path within the objective are also disadvantageous. Even slight contamination at the interfaces and in the material of the optical components that are used can adversely mount up, sharply reduce the intensity of the light and generate interfering scattered light.
An arrangement with tilted lenses is also described in U.S. Pat. No. 4,730,910.
Further, fundus cameras are known which have mirror elements in place of the opthalmoscope lens. These systems have simple mirror geometries with which only a small observation field and illumination field can be implemented with adequate optical quality.
Other mirror systems such as those described, e.g., in U.S. Pat. No. 6,585,374, use moving elements to expand the small observation field and illumination field based on scanning principles. However, such systems need complicated mechanisms for precise movement of the elements in question and require complicated image processing techniques.